The conductive elements that connect electronic components on a circuit board and couple signals around a circuit board may be produced by "writing" conductive traces on the board material with a conductive polymeric composition. Typically, the polymeric material comprises a conductive filler which is dispersed in a resin formulation that includes solvents to reduce viscosity. The resulting conductive polymeric material is extruded in its uncured state through a syringe-like apparatus which is translated over the circuit board to deposit conductive traces in the desired circuit pattern or screen printed through a pattern created by a stencil. For certain known conductive polymeric materials, traces formed of such materials are periodically cured by heating the material to evaporate the solvents.
Ideally, conductive polymeric compositions are cured quickly and thoroughly so that insulator layers and additional conductive traces can be added directly over the cured conductive traces without causing short circuits. In addition, the conductive traces must maintain good electrical contact with the metal pads to which electrical components and leads are connected. The material should also maintain its conductive properties through the curing process. This is particularly important where the conductive traces from arcuate sections, such as curves and `hops` over previously deposited traces. The arcuate geometry creates uneven stresses in the conductive traces which may be exacerbated by curing. Further, it is often necessary to deposit the conductive material in vias, which are narrow diameter passages through the circuit board for connecting circuitry on opposite sides of the board. Suitable materials must be readily cured despite the limited exposed surface area available for solvent evaporation in vias.
To achieve the desired conductivity while maintaining the low viscosities necessary for extruding circuit traces, conventional conductive polymeric materials contain solvents. Solvents reduce the viscosity of conventional conductive polymeric materials and increase the concentration of conductive filler material when they evaporate following deposition.
The solvent content of conventional conductive polymeric materials limits their effectiveness for writing circuits in the manner described above, since the loss of solvent alters the characteristics of the conductive traces. For example, conventional solvent based materials may shrink up to 50% on curing. This dimensional instability creates cracks in the conductive traces, particularly along arcuate sections and hops, and the conductive trace may break completely if its volume changes sufficiently. Similarly, contacts with metal pads are easily broken as the shrinking resin volume pulls the conductive trace away from the pad, and the conductive polymeric material can be blown out of vias by escaping solvent.
Solvent evaporation causes a number of problems in addition to those associated with dimensional instability. For example, evaporation of solvents prior to curing changes the composition of the conductive polymer materials, and leads to inconsistencies in the material viscosity from batch to batch. Further, evaporating solvents may leave air pockets in the material, altering its physical properties in unpredictable ways.
It is thus desirable to develop dimensionally stable conductive polymeric materials having uniform consistencies and the low viscosities necessary for depositing narrow diameter conductive traces. Most commercially available UV curable resins are inadequate for circuit writing applications. For example, UV cured acrylates have poor adhesion to metals. Also, UV cure of acrylates is oxygen inhibited, resulting in a tacky surface due to incomplete curing. Further, silver which is the standard conductive filler material, may catalyze polymerization of many formulations at room temperature before exposure to UV radiation. Once polymerized, these acrylate formulations cannot be used to write circuit patterns.